Wafer testing systems typically include a test head and a probe card. The probe card includes a pattern of contacts for electrically probing portions of an integrated circuit formed on a semiconductor wafer. The test head is configured to drive various contacts of the probe card to carry out particular test procedures within the integrated circuit. In the course of a test procedure, the test head receives output signals from the integrated circuit via the contacts of the probe card. The output signals are indicative of electrical characteristics of the integrated circuit under test. The probe card and test head are uniquely configured for a particular integrated circuit and, in some cases, a particular test procedure. Accordingly, the probe card and/or test head must be changed for different integrated circuits and test procedures.
The test head and probe card are electrically coupled to one another with a coupling apparatus often referred to as a "pogo" unit. The pogo unit engages the test head, or some intermediate coupling structure associated with the test head, and the probe card. The pogo unit includes an array of spring-loaded contacts referred to as pogo pins. The pogo pins are arranged to electrically couple contacts on the probe card to corresponding contacts on the test card. The spring force of the pogo pins helps maintain uniformity of electrical contact between the various contacts of the probe card and test head. When the test head and probe card are engaged with the pogo unit to exert pressure against the pogo pins, the pins respond with a spring force that enhances coupling pressure. The resilience of the pins generally ensures adequate coupling pressure despite planar deformation of the test head or probe card during a test procedure.